1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a diffuser sheet and an illumination apparatus, and more particularly, to a diffuser sheet that can compensate for chromatic dispersion generated in a hologram light guide plate, and an illumination apparatus for a display employing the diffuser sheet.
2. Description of the Related Art
Non-emissive displays, such as liquid crystal displays (LCDs), typically require a separate illumination apparatus, such as a backlight unit. FIG. 1 shows an exemplary conventional illumination apparatus for a display provided with a light guide plate using a hologram. Referring to FIG. 1, the conventional illumination apparatus 10 includes a light guide plate 12 having a fine hologram pattern 13 formed on an upper surface thereof, a light source 11 disposed adjacent to one sidewall of the light guide plate 12, and a diffuser sheet 15 which uniformly diffuses the light emitted from the upper surface of the light guide plate 12.
The light source 11 of the illumination apparatus may be, for example, a white light emitting diode (LED). White light emitted from the light source 11 is incident obliquely through one sidewall surface of the light guide plate 12. The light guide plate 12 is made of poly methyl meth Acrylate (PMMA) which is a plastic material with a superior transmittance. The white light within the light guide plate is totally internally reflected by the upper and lower surfaces of the light guide plate. As shown in FIG. 1, since the light guide plate 12 has a hologram pattern 13 formed on the upper surface thereof, some of the light incident on the upper surface of the light guide plate 12 is diffracted by the hologram pattern 13 and is emitted through the upper surface of the light guide plate 12. For example, the hologram pattern 13 may have a sinusoidal form with a predetermined period. The light emitted through the upper surface of the light guide plate 12 is uniformly diffused through the diffuser sheet 15 to illuminate a non-emissive display, such as an LCD.
Refractive indices and transmittance vary depending on the wavelength of the light. Thus, when white light is emitted through the upper surface of the light guide plate 12 and through the hologram pattern 13, chromatic dispersion takes place. FIGS. 2A through 2C illustrate this chromatic dispersion and show measurement results of brightness distributions for red(R), green (G) and blue (B) light emitted from the hologram pattern 13. In the illustrated examples, the hologram pattern 13 has a period of 420 nm and a thickness of 250 nm. Also, the blue light has a wavelength of 600 nm, the green light has a wavelength of 555 nm and the blue light has a wavelength of 450 nm. FIG. 2A illustrates a brightness distribution of red light. As shown in FIG. 2A, most of the red light is distributed near incident center. FIG. 2B illustrates a brightness distribution of green light. As shown in FIG. 2A, the green light is distributed near the incident center, but the brightest portion slightly deviates from the incident center. FIG. 2C illustrates a brightness distribution of blue light. As shown in FIG. 2C, the brightest portion greatly deviates from the incident center. As shown, the brightness distribution is different according to the color of light.
As shown in FIG. 3, chromatic dispersion can be clearly seen from the brightness distributions of red (R), green (G) and blue (B) light. The angles shown in FIG. 3 are value measured with respect to the normal line perpendicular to the light guide plate 12. As shown in FIG. 3, the blue light is distributed centering on the angle of 22°, which is called a peak angle. In other words, the blue light is emitted from the light guide plate 12 at a peak angle of 22°. The green light is emitted from the light guide plate 12 at a peak angle of 8°. The red light is emitted from the light guide plate 12 at a peak angle of 4°.
Thus, the angle of the light emitted from the light guide plate 12 is different depending on wavelength causing the problem that the white light emitted from the light source 11 is divided into respective colors by the light guide plate 12. In the conventional art, the diffuser sheet 15 serves to uniformly diffuse the incident light, but it is limited in its ability to completely reproduce the divided color lights to white light. Therefore, display performance of the display is lowered.